Motor driving devices that drive a motor by windings of multiple phases are conventionally known. In the electric power steering device disclosed in JPA-2009-6963, which corresponds to US 2010/0017063 A1, for example, a three-phase brushless motor is used and when any anomaly occurs in one phase, control is carried out to continue driving of the motor by the remaining two phases.
In general, any failure is detected in switching elements, a pre-driver, and the like before starting driving of a motor so that the motor will not be broken. As an example, it will be assumed that the inverter in each system is driven with setting so made that the rate of on and off of the high potential-side switching elements and the low potential-side switching elements is 1:1 in the inverter. When each inverter is normal, the terminal voltage is ½ of the voltage applied immediately before the inverter. When any failure has occurred, the terminal voltage is other than ½ of the voltage applied immediately before the inverter. Any failure in the inverters can be determined by utilizing this, for example.
However, when the above-mentioned so-called PWM driving is carried out in inverter failure detection, a problem may arise. When a failure (hereafter, referred to as “short circuit failure”) in which any switching element cannot be brought out of conduction (non-conduction) has occurred, an overcurrent flows from a high potential-side switching element to a low potential-side switching element and this burns an inverter.